The dehydrogenation of an organic compound to produce an unsaturated compound from a saturated organic compound, or to produce a more highly unsaturated organic compound from an already unsaturated compound, is a widely used industrial chemical process. For example, ethylbenzene or propylbenzene can be dehydrogenated to produce styrene and 2-methylstyrene, respectively, and the styrene monomers so-produced are useful for preparing polymeric compositions, e.g. polystyrenes. Linear alkanes can also be dehydrogenated to produce alkenes and these alkenes can be used to alkylate aromatic compounds. For example, benzene or toluene can be alkylated with high molecular weight linear olefins to produce alkylated intermediates useful for the preparation of surfactants and detergents. More recently, however, there has been interest in the preparation of dialkylnaphthalene compounds, for example, 2,6-dimethylnaphthalene, by the dehydrogenation of a dimethyltetralin. Dialkylnaphthalenes, and particularly 2,6-dimethylnaphthalene, are suitable feedstocks for oxidation to naphthalenedicarboxylic acid. 2,6-Naphthalenedicarboxylic acid, for example, is a monomer useful for preparing high performance polymers such as polyesters.
One suitable synthetic route for the preparation of 2,6-dimethylnaphthalene comprises reacting o-xylene with butadiene in the presence of a alkali metal catalyst to form 5-ortho-tolylpentene (5-OTP). The 5-OTP is cyclized to form 1,5-dimethyltetralin (1,5-DMT), which is then dehydrogenated to form 1,5-dimethylnaphthalene (1,5-DMN). The 1,5-dimethylnaphthalene can be isomerized to 2,6-dimethylnaphthalene (2,6-DMN). A process for preparing 5-OTP from o-xylene and butadiene is disclosed in, for example, U.S. Pat. No. 3,953,535 to Shima, et al. The cyclization process, which is depicted by the following equation, is disclosed in U.S. Pat. Nos. 5,030,781 and 5,034,561 to Sikkenga, et al. ##STR1##
The dehydrogenation reaction where 1,5-DMT is converted to 1,5-DMN is typically conducted at elevated temperatures in the presence of a solid, heterogeneous dehydrogenation catalyst. The reaction is illustrated by the following equation: ##STR2##
In order to make the overall process for converting o-xylene and butadiene to 2,6-dimethylnaphthalene acceptable for commercial-scale production, particularly because of the number of process steps, it is essential to have each reaction step be as selective as possible and to proceed in as high a yield as possible. Therefore, it is important to use a dehydrogenation catalyst that can catalyze the conversion of 1,5-DMT to 1,5-DMN in a highly selective and high yield manner.
A process for preparing dimethylnaphthalene from dimethyltetralins using heterogeneous catalysts is disclosed, for example, in U.S. Pat. No. 3,775,496 to Thompson, wherein a dimethyltetralin is dehydrogenated to a dimethylnaphthalene using a platinum on alumina catalyst. In the process disclosed therein, the dimethyltetralin is contacted with the solid dehydrogenation catalyst at a temperature in the range of about 300.degree.-500.degree. C. in the presence of hydrogen. It is also disclosed that it is preferable to use a platinum on non-acidic alumina catalyst. This patent also teaches that U.S. Pat. No. 3,325,551 to Suld discloses a method for preparing non-acidic dehydrogenation catalysts. In the method disclosed in the Suld U.S. Pat. No. 3,325,551 patent, a basic alkali metal salt, preferably lithium carbonate, is deposited on the catalyst. For example, an alumina support having platinum metal deposited thereon is saturated with an aqueous lithium carbonate solution after which the wet catalyst is heated to, for example, 150.degree.-260.degree. C., in order to remove the water. Methods for dehydrogenating dimethyltetralins to dimethylnaphthalenes are also disclosed in Sikkenga, et al. U.S. Pat. application Ser. No. 556,350, filed Jul. 20, 1990, now U.S. Pat. No. 5,118,892.
Methods for preparing non-acidic dehydrogenation catalysts are known in the art. For example, U.S. Pat. No. 4,268,707 to Antos teaches a method for preparing a non-acidic catalytic composition comprising a porous carrier material containing 0.01 to about 2 wt. % platinum group metal, about 0.05 to about 5 wt. % cobalt, about 0.1 to about 5 wt. % alkali metal or alkaline earth metal, and about 0.01 to about 5 wt. % lanthanide series metal. It is disclosed in the Antos patent that the alkali or alkaline earth metal component is added to neutralize any acidic material such as halogen which may have been present in the preparation of the catalyst so that the final catalyst is non-acidic. U.S. Pat. No. 3,531,543 to Clippinger et al. discloses catalyst compositions comprising a Group VIII noble metal component, tin and an inorganic refractory metal oxide carrier. The Clippinger et al. patent discloses that halogen can be removed from the composite prior to calcination by elutriation, ion-exchange, steaming, etc. British Patent Specification 1,499,297 discloses a method and a catalyst for dehydrogenating paraffin hydrocarbons. The catalysts disclosed therein comprise a carrier of active alumina having deposited thereon platinum in an amount of from 0.2 to 1.0% by weight, an alkali metal in an amount of from 0.2 to 2.0% by weight, and at least one of the three elements gallium, indium and thallium in a total amount of from 0.2 to 1.0% by weight, all percentages being with respect to the weight of the catalyst. This British Patent Specification also discloses that when a halogen is incorporated during the catalyst preparation, for example, by impregnating the carrier with chlorine-containing compounds of platinum, the impregnated, dried and calcined carrier should be subjected to a treatment to reduce the halogen content in the catalyst. It is taught that the impregnated, dried and calcined carrier may be treated with an aqueous solution of ammonia at a temperature of from 50.degree. to 90.degree. C. to reduce the halogen content in the catalyst to from 0.01 to 0.1% by weight.
The art, however, needs improved processes and catalysts that can be used for the selective dehydrogenation of organic compounds and particularly for the dehydrogenation of dimethyltetralins to dimethylnaphthalenes. The present invention provides such improved processes and catalysts.